I'd like to start this off by saying I am really young and have never had to deal with taxes, I'm also new to Bitcoin.

So lets say I get paid under the table (off the books) in Bitcoin, could I simply claim that the Bitcoin was mined by me, pay capital gain tax (or equivalent of that in my country) for selling the Bitcoin, and be safe? There's no way to prove I mined the Bitcoin myself (especially if I claim I do not have access to the wallet anymore), surely I can't get in trouble for that?